Piezoresistive pressure sensors differ from other such sensors, particularly from piezoelectric pressure sensors, in that they are able to measure pressures reliably over very long periods of time. Piezoelectric pressure sensors need to be “reset” in order to be made ready to start taking measurements again, because over time they lose their charge and “drift”.
Examples of absolute pressure sensors are piezoresistive sensors, particularly oil-filled piezoresistive pressure sensors. These comprise a sensor chip element that is placed on a substrate or on a passthrough. As a rule, this is attached with an adhesive. Finally, oil is passed round the sensor chip element under a membrane. If a pressure is applied outside of the membrane, the oil under the membrane is also subjected to pressure. The sensor chip under pressure generates a corresponding signal, which is finally forwarded to an evaluation unit via two more contacts with connecting wires. It is quite possible that such sensor may also be constructed without membranes. The sensor chip is then exposed to the external pressure medium directly.
It has been discovered that over time, signal drift also occurs with the sensors described here. This means that the pressure indicated changes slightly with time for the same load. This drift is of a much smaller order than occurs with piezoelectric pressure sensors, because it has a different physical cause. Piezoelectric elements lose their charges over time, piezoresistive elements do not. The drift in piezoresistive pressure sensors is in the order of about 0.1%.
The pressure is exerted evenly on the surface of the substrate or the passthrough on which the sensor chip element is positioned centrally. Particularly under high pressure of more than 50 bar, the passthrough sags somewhat, so that a small recess is created under the middle of the sensor chip element. It has been discovered that after a period the adhesive between the passthrough and the sensor chip element creeps inwards to fill this recess. Moreover, the adhesive may also be buckled by the pressure in the edge area of the sensor chip element and the substrate, causing the sensor chip element to be deformed. After some time, the creep of the adhesive compensates for this buckling. This results in signal drift, because the conditions of counterpressure from below the sensor chip element vary over time.
In order to correct this problem, in some cases the adhesive was dispensed with. Then, the sensor chip element was attached only by the contacts. However, these contacts were sometimes destroyed by vibrations and the sensor was unable to transmit any measurement values.
A rectangular sensor chip element that is fastened to a substrate with a drop of adhesive at each of the four lower corners is known from U.S. Pat. No. 6,543,292. This is intended to prevent thermal tensions between the substrate and the chip. However, it has been discovered that the capillary effect in the gap between the sensor chip element lower face and the substrate upper face causes the adhesive to disperse in this gap, either as it is being applied, or during subsequent use. This can cause the adhesive to spread over a large area of the gap, in some cases over the entire area of the gap. The sizes of the surfaces covered by the adhesive drops cannot be checked. The problem of signal drift described earlier is thus not eliminated; the sensor chip elements can be buckled in this application as well.
Assembly is difficult even when highly viscous adhesives exhibiting hardly any capillary effect are used. The pressure with which sensor chip element 2 is positioned on the adhesive cannot be controlled completely. Thus, the adhesive is squeezed under element 2 in uncontrolled manner, and it is not possible to determine how far the adhesion area reaches towards the middle of element 5.
Apart from the absolute pressure sensors to which this invention relates, differential pressure sensors are also known. Unlike the absolute pressure sensors, the chip cavity in the differential pressure sensors is not closed, but it is in a pressure connection with a second pressure medium. Since the element calculates the differential pressure between the ambient pressure and the cavity pressure, the substrate itself is not exposed to a load. Such an example is described in JP 61-226627. The element is attached with terminals, which hold it in place by spring force. Since no adhesive is used, the element is also not buckled.
FIG. 1 is a diagrammatic representation of a pressure sensor 1 according to the prior art. A piezoresistive sensor chip element 2 is located in a housing 12 and is attached by the element lower face 5 thereof to a substrate 6. In this variation, sensor chip element 2 comprises a piezoresistive chip 3 on a block-shaped chip base 4. A chip cavity 27 is included between said chip 3 and base 4. Chip 3 measures the respective pressure difference between the reference pressure in chip cavity 27 and external pressure acting on chip 3. A pressure medium 14 flows around sensor chip element 2 on all sides except the underside 5 thereof and by exerting a pressure generates a measurement signal that is forwarded by contacts 25. Said contacts pass through substrate 6, which in this case is designed as a passthrough. An insulating element 11 provides a seal for the pressure chamber filled with pressure medium 14. Finally, the measurement signals are processed in an evaluation unit not shown here.
In this embodiment, housing 1 is closed off from pressure chamber 26 by a membrane 13. In this way, contacts 25 and sensor chip element 2 are protected against mechanical and chemical influences from pressure chamber 26. In these variations, the space around sensor chip element 2 is usually filled with the oil pressure medium 14, which is always under the same pressure as pressure chamber 26 due to the soft membrane 13. Other, equivalent variations do not have a membrane 13. Consequently, sensor chip element 2 is in direct contact with the pressure medium 14 of pressure chamber 26.
Sensor chip element 2 has an element lower face 5, which is located opposite chip 3 on chip base 4. This element lower face 5 is positioned on face 7 of substrate 6, which faces towards pressure chamber 26. In this embodiment according to the prior art, adhesion area 8, by which sensor chip element 2 is fastened to substrate 6, occupies the entire surface area of element lower face 5. An adhesive mass 24 is usually used to ensure adhesion.
FIG. 2 represents a cross section of a known sensor chip element 2 according to FIG. 1 on a substrate 6. Adhesive substance 24 is applied evenly between the element lower face and the substrate upper face. This FIG. 2 represents an arrangement without pressure loading.
FIGS. 3a and 3b shows the same prior art sensor chip element 2 of FIG. 2 under pressure load at the time the pressure is applied (FIG. 3a) and a long time later (FIG. 3b). Since the present invention relates to long-duration pressure sensors that are able to take measurements reliably for many months or years without requiring a “Reset”, the time difference between such two representations may be correspondingly long.
The arrows around sensor chip element 2 in FIGS. 3a and 3b show the pressure load from pressure medium 14 that is acting thereon. In both FIGS. 3a and 3b, substrate 6 sags under the applied pressure, resulting in a curvature of substrate upper face 7.
In FIG. 3a, when the pressure is first applied, the pressure load on element lower face 5 is lower in the centre because adhesive substance 24 draws this area toward substrate upper face 7. Accordingly, the sensor chip element is slightly deformed, which results in a slight increase in the measured value calculated by chip 3. A dashed line on the chip is an exaggerated representation of this sagging.
The pressure also acts laterally on adhesive substance 24. In combination with the negative pressure created centrally below element lower face 5, over time adhesive substance 24 slowly creeps towards the centre, as is shown in FIG. 3b. This causes the pressure on element lower face 5 to change, and therewith also the measurement signal, even though the pressure is unchanged. The pressure on sensor chip element 2 is reduced, and the element tends towards the shape it had before pressure was applied as in FIG. 3. The arrows of equal length along a cross section of the sensor chip element indicate this correspondingly.
However, as soon as the pressure in the pressure chamber falls to the ambient pressure and the substrate regains its former shape, the adhesive, which has meanwhile accumulated in the centre, exerts increased pressure on the sensor chip element, thereby generating a false signal, which leads to the incorrect conclusion that pressure has increased in the pressure chamber.